What's your question? Ask

There are several 'rots' or fungal infections that calla lilies are prone to. You may have over watered them or the bulbs may have been infected before planting. There's nothing you can do. Don't replant your new bulbs in the same spot.

Your Calla Lily is normal and you're doing the right thing. It doesn't hurt the plant to cut the flower off. It puts more energy into the bulb rather than the seed head -the hard center.

This normally happens when they are not getting quite enough light. Is it possible that thei light conditions have changed in the past year or so, perhaps a tree branch that has grown out or a new structure that was put up near it or even a shrub that has grown up and is blocking the light?

The other less likely possibility is that they need nutrients. Fertilize the plants if you have not done so this year.

You can cut back the leaves when they turn yellow.

This article (bottom section) will help you with what to do with them:
https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm

They may need more light or it may be that they flowers have not aged enough. Calla flowers normally start off green, or with a green tinge but will lose the green as the flower matures.

Yes, it could be too much nitrogen. You can counteract this by adding some phosphorus rich fertilizer or bone meal to the soil.